A Single phase induction motors (SPIM) need an auxiliary winding in parallel with motor main winding for starting. Also, there should be a current phase difference between the main and auxiliary windings of SPIMs. This is usually facilitated by inserting a capacitor in series with the auxiliary winding. The auxiliary winding and the capacitor may also improve the motor performance if they are not disconnected from the supply after starting. Such a capacitor is preferred to be adjusted to provide a close to 90° phase differences between currents of the two windings. This results in a balanced motor operation. However, in variable speed drives, the 90° can not be maintained at non-rated speeds unless the capacitor impedance is readjusted to an optimized value at each supply frequency by additional means. Therefore, the balanced operation can not be provided and the motor efficiency reduces without capacitor readjustment.
Variable speed drives improve energy efficiency. If they are implemented with efficiency optimizing schemes, even more energy saving is gained. Various variable speed drives and efficiency optimization schemes have been proposed for SPIMs. U.S. Pat. No. 4,455,521 has presented a circuit for controlling the operation of a SPIM to obtain maximum motor efficiency over an entire range of load conditions. It has used a well known stator voltage control method to reduce motor flux when motor load decreases. This is a constant frequency control and doesn't provide variable speed capability for motor. Similar methods of stator voltage control for constant speed applications have been used in U.S. Pat. No. 4,409,528 and U.S. Pat. No. 5,670,858.
A single phase sinusoidal shaped variable voltage variable frequency waveform has been proposed in U.S. Pat. No. 4,706,180 for driving a single phase ac induction motor by employing a sinusoidal pulse width modulated signal to switch a pair of solid state power switches. In U.S. Pat. No. 5,252,905 a variable frequency driving system has additional options: an ac power line for providing single phase ac power line voltage at a fixed frequency to drive the motor at an adjustable speed. The last two patents concentrate on the improvement of power supply specifications. However, control of motor and improvement of its performance are not considered. In U.S. Pat. No. 4,566,289, a refrigerator control system has been implemented by a change-over device to drive a motor both through an inverter at reduced speed and directly by a commercial power supply at a nominal speed. As a result, the inverter loss is eliminated when it is bypassed and the total efficiency of the refrigerator increases. This idea has been used in U.S. Pat. No. 6,570,778 B2 in a different way such that the main winding of the motor is supplied directly from ac power line to develop start-up torque; while in lower speed values the motor is supplied by an inverter. In the last two patents, motor efficiency may decrease substantially at non-rated frequencies when the motor is supplied from the inverter or under non-rated loads.
In the present invention a low cost variable speed drive for SPIMs is proposed to control motor speed and maximize motor efficiency over the entire speed range.